In the aerospace field and energy and environmental field, for the purposes of improving efficiency and performance, attention has been paid to SiC-based heat-resistant materials that are excellent in oxidation resistance, remain stable for a long time under a high temperature, and have corrosion resistance, high heat conductivity, small thermal expansion property, and low specific densities. Application of such SiC-based materials to a combustor component, a heat exchanger, etc., which are required to be made from a dense material, is also being considered. Representative SiC-based materials include monolithic SiC ceramics, SiC fiber-reinforced SiC composite materials (hereinafter referred to as SiC/SiC), and SiC fiber-bonded ceramics.
Monolithic SiC ceramics are hard and dense and have excellent heat resistance. Therefore, they are used for a sliding member such as a ball bearing, a sliding bearing, etc. in a high-temperature region and a cryogenic region in which a lubricant cannot be used. However, monolithic SiC ceramics lack reliability because they are brittle, being sensitive to minute defects.
On the other hand, SiC/SiC is a material that overcomes the brittleness of monolithic SiC ceramics with a toughening mechanism such as fiber bridging, crack deflection, etc. SiC/SiC is produced mainly by CVI (Chemical Vapor Infiltration) method, PIP (Polymer Infiltration and Pyrolysis) method, and MI (Melt Infiltration) method. However, since all of these methods leave pores in the material, the material needs to be coated with a dense surface layer in order to be applied to a component required to have denseness. Hence, as a process for producing dense SiC/SiC, Patent Document 1 discloses a process for producing a high-density SiC fiber-reinforced SiC composite material by hot pressing. This material is produced by preparing a slurry containing dispersed SiC fine powder and sintering aid, making a preform by impregnating the slurry into SiC fiber coated with one or two or more of carbon, boron nitride, and silicon carbide, and hot-pressing the preform at a sintering temperature of 1600 to 1800° C. and at a pressure of 10 MPa or higher.
Meanwhile, SiC fiber-bonded ceramics are produced by hot-pressing only amorphous Si-M-C-O fiber (M being at least one or more metal element among group IIA, group IIIA, and group IIIB metal elements). In the process for producing SiC fiber-bonded ceramics, the amorphous fiber structurally changes to polycrystalline SiC fiber and at the same time to a closest-packed hexagonal column under a high temperature and a high pressure while generating gas. In the process of the structural changes of the fiber, excess carbon in the amorphous fiber is eliminated onto the fiber surface and formed into a layer structure on the fiber surface. Since this carbon layer on the fiber surface functions as a sliding layer that deflects crack propagation, SiC fiber-bonded ceramics exhibit excellent fracture toughness. From these facts, SiC fiber-bonded ceramics are a material that overcomes the brittleness of monolithic ceramics and the insufficient denseness of SiC/SiC. Particularly, Patent Document 2 discloses a process for hot-pressing a material via a pressure-transmitting medium made of inorganic powder in order to produce SiC fiber-bonded ceramics having a complex shape.
Patent Document 3 discloses a process for producing a bar-shaped or tubular fiber-reinforced ceramics composite material by hot isostatic pressing. According to patent Document 3, the process for producing a fiber-reinforced ceramics composite material includes making a preform by forming prepreg sheet made of inorganic fiber and ceramics powder or forming inorganic fiber made of an inner layer and a surface layer into a certain shape, sealing the preform in a glass capsule, and hot-isostatic-pressing the encapsulated preform. In this process, there is a step of covering the surface of the preform with a substance that remains stable by reacting with none of the preform and the glass and does not allow permeation of the glass thereinto. The viscosity of the glass capsule during hot isostatic pressing is 103 to 107.5 P. The amount of glass used in the hot isostatic pressing process is adjusted in a way to release any residual tensile stress that is generated on the product due to breakage of the glass of the glass capsule covering the surface of that substance during a cooling process. The inorganic fiber recited in claim 1 of Patent Document 3 is thermally very stable and can be considered to keep the structure before the hot isostatic pressing process even after the process. The inorganic fiber made of an inner layer and a surface layer recited in claim 2 of Patent Document 3 also does not have a large structural change after the hot isostatic pressing process, though with a slight structural change. In this inorganic fiber, the inner layer is made of (a) an amorphous material substantially consisting of Si, M, C, and O (where M being Ti or Zr), (b) an assembly of crystalline superfine particle substantially consisting of at least one or more of β-SiC, MC, a solid solution of β-SiC and MC, and MC1-X, and C; and an amorphous material consisting of SiO2 and MO2 (where X being a number not less than 0 and less than 1), or (c) a mixture of the amorphous material of (a) and the assembly of (b). The surface layer is made of (d) an amorphous material substantially consisting of Si, M, and O, (e) a crystalline assembly consisting of crystalline SiO2 and MO2, or (f) a mixture of the amorphous material of (d) and the crystalline assembly of (e). That is, neither of these inorganic fibers generates a large amount of gas before or after hot isostatic pressing.